<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #frozen-btn {
      display: flex;
      align-items: center;
      justify-content: center;
      height: 100vh;
    }

    button {
      border: 0;
      margin: 20px;
      text-transform: uppercase;
      font-size: 20px;
      font-weight: bold;
      padding: 15px 50px;
      border-radius: 50px;
      color: white;
      outline: none;
      position: relative;
    }

    button:before {
      content: '';
      display: block;
      background: linear-gradient(to left, rgba(255, 255, 255, 0) 50%, rgba(255, 255, 255, 0.4) 50%);
      background-size: 210% 100%;
      background-position: right bottom;
      height: 100%;
      width: 100%;
      position: absolute;
      top: 0;
      bottom: 0;
      right: 0;
      left: 0;
      border-radius: 50px;
      transition: all 1s;
      -webkit-transition: all 1s;
    }

    .green {
      background-image: linear-gradient(to right, #25aae1, #40e495);
      box-shadow: 0 4px 15px 0 rgba(49, 196, 190, 0.75);
    }

    .purple {
      background-image: linear-gradient(to right, #6253e1, #852D91);
      box-shadow: 0 4px 15px 0 rgba(236, 116, 149, 0.75);
    }

    .purple:hover:before {
      background-position: left bottom;
    }

    .green:hover:before {
      background-position: left bottom;
    }
  </style>
</head>

<body>
  <div id="frozen-btn">
    <button class="green">Hover me</button>
    <button class="purple">Hover me</button>
  </div>

</body>

</html>